On Sept. 9, 1969, a big brown envelope was delivered to the Oval Office on behalf of CIA Director Richard M. Helms. On it he had written, "For and to be opened only by: The President, The White House." The precise contents of the envelope are still unknown, but it was the latest intelligence on one of Washington's most secretive foreign policy matters: Israel's nuclear program. The material was so sensitive that the nation's spymaster was unwilling to share it with anybody but President Richard M. Nixon himself.

The now-empty envelope is inside a two-folder set labeled "NSSM 40," held by the Nixon Presidential Materials Project at the National Archives. (NSSM is the acronym for National Security Study Memorandum, a series of policy studies produced by the national security bureaucracy for the Nixon White House.) The NSSM 40 files are almost bare because most of their documents remain classified.

With the aid of With the aid ofrecently declassified documents , we now know that NSSM 40 was the Nixon administration's effort to grapple with the policy implications of a nuclear-armed Israel. These documents offer unprecedented insight into the tense deliberations in the White House in 1969 -- a crucial time in which international ratification of the nuclear Non-Proliferation Treaty (NPT) was uncertain and U.S. policymakers feared that a Middle Eastern conflagration could lead to superpower conflict. Nearly four decades later, as the world struggles with nuclear ambitions in Iran, India and elsewhere, the ramifications of this hidden history are still felt.

Israel's nuclear program began more than 10 years before Helms's envelope landed on Nixon's desk. In 1958, Israel secretly initiated work at what was to become the Dimona nuclear research site. Only about 15 years after the Holocaust, nuclear nonproliferation norms did not yet exist, and Israel's founders believed they had a compelling case for acquiring nuclear weapons. In 1961, the CIA estimated that Israel could produce nuclear weapons within the decade.

The discovery presented a difficult challenge for U.S. policymakers. From their perspective, Israel was a small, friendly state -- albeit one outside the boundaries of U.S. security guarantees -- surrounded by larger enemies vowing to destroy it. Yet government officials also saw the Israeli nuclear program as a potential threat to U.S. interests. President John F. Kennedy feared that without decisive international action to curb nuclear proliferation, a world of 20 to 30 nuclear-armed nations would be inevitable within a decade or two.

The Kennedy and Johnson administrations fashioned a complex scheme of annual visits to Dimona to ensure that Israel would not develop nuclear weapons. But the Israelis were adept at concealing their activities. By late 1966, Israel had reached the nuclear threshold, although it decided not to conduct an atomic test.

By the time Prime Minister Levi Eshkol visited President Lyndon B. Johnson in January 1968, the official State Department view was that despite Israel's growing nuclear weapons potential, it had "not embarked on a program to produce a nuclear weapon." That assessment, however, eroded in the months ahead. By the fall, Assistant Defense Secretary Paul C. Warnke concluded that Israel had already acquired the bomb when Israeli Ambassador Yitzhak Rabin explained to him how he interpreted Israel's pledge not to be the first country to introduce nuclear weapons into the region. According to Rabin, for nuclear weapons to be introduced, they needed to be tested and publicly declared. Implicitly, then, Israel could possess the bomb without "introducing" it.

The question of what to do about the Israeli bomb would fall to Nixon. Unlike his Democratic predecessors, he and his national security adviser, Henry A. Kissinger, were initially skeptical about the effectiveness of the NPT. And though they may have been inclined to accommodate Israel's nuclear ambitions, they would have to manage senior State Department and Pentagon officials whose perspectives differed. Documents prepared between February and April 1969 reveal a great sense of urgency and alarm among senior officials about Israel's nuclear progress.

As Defense Secretary Melvin R. Laird wrote in March 1969, these "developments were not in the United States' interests and should, if at all possible, be stopped." Above all, the Nixon administration was concerned that Israel would publicly display its nuclear capabilities.

Apparently prompted by those high-level concerns, Kissinger issued NSSM 40 -- titled Israeli Nuclear Weapons Program -- on April 11, 1969. In it he asked the national security bureaucracy for a review of policy options toward Israel's nuclear program. In the weeks that followed, the issue was taken up by a senior review group (SRG), chaired by Kissinger, that included Helms, Undersecretary of State Elliot Richardson, Deputy Defense Secretary David Packard and Joint Chiefs Chairman Earle Wheeler.

The one available report of an SRG meeting on NSSM 40 suggests that the bureaucracy was interested in pressuring Israel to halt its nuclear program. How much pressure to exert remained open. Kissinger wanted to "avoid direct confrontation," while Richardson was willing to apply pressure if an investigation to determine Israel's intentions showed that some key assurances would not be forthcoming. In such circumstances, the United States could tell the Israelis that scheduled deliveries of F-4 Phantom jets to Israel would have to be reconsidered.

By mid-July 1969, Nixon had let it be known that he was leery of using the Phantoms as leverage, so when Richardson and Packard summoned Rabin on July 29 to discuss the nuclear issue, the idea of a probe that involved pressure had been torpedoed. Although Richardson and Packard emphasized the seriousness with which they viewed the nuclear problem, they had no threat to back up their rhetoric.

Richardson posed three issues for Rabin to respond to: the status of Israel's NPT deliberations; assurances that "non-introduction" meant "non-possession" of nuclear weapons; and assurances that Israel would not produce or deploy the Jericho ballistic missile. Rabin, however, was unresponsive except to say that the NPT was still "under study."

Nixon and Israeli Prime Minister Golda Meir would have to address the nuclear issue when they met in late September.

Perhaps the most fateful event of this tale was Nixon's one-on-one meeting with Meir in the Oval Office on Sept. 26, 1969.

In the days before Meir's visit, the State Department produced background papers suggesting that the horse was already out of the barn: "Israel might very well now have a nuclear bomb" and certainly "already had the technical ability and material resources to produce weapon-grade material for a number of weapons." If that was true, it meant that events had overtaken the NSSM 40 exercise.

In later years, Meir never discussed the substance of her private conversation with Nixon, saying only, "I could not quote him then, and I will not quote him now." Yet, according to declassified Israeli documents, since the early 1960s, Meir had been convinced that "Israel should tell the United States the truth [about the nuclear issue] and explain why."

Even without the record of this meeting, informed speculation is possible. It is likely that Nixon started with a plea for openness. Meir, in turn, probably acknowledged -- tacitly or explicitly -- that Israel had reached a weapons capability, but probably pledged extreme caution. (Years later, Nixon told CNN's Larry King that he knew for certain that Israel had the bomb, but he wouldn't reveal his source.) Meir may have assured Nixon that Israel thought of nuclear weapons as a last-resort option, a way to provide her Holocaust-haunted nation with a psychological sense of existential deterrence.

Subsequent memorandums from Kissinger to Nixon provide a limited sense of what the national security adviser understood happened at the meeting. Kissinger noted that the president had emphasized to Meir that "our primary concern was that the Israeli [government] make no visible introduction of nuclear weapons or undertake a nuclear test program." Thus, Israel would be committed to conducting its nuclear affairs cautiously and secretly; their status would remain uncertain and unannounced.

On Feb. 23, 1970, Rabin told Kissinger privately that he wanted the president to know that, in light of the Meir-Nixon conversation, "Israel has no intention to sign the NPT." Rabin, Kissinger wrote, "wanted also to make sure there was no misapprehension at the White House about Israel's current intentions."

Kissinger informed Nixon that he told Rabin that he would notify the president. And with that, the decade-long U.S. effort to curb Israel's nuclear program ended. That enterprise was replaced by understandings negotiated at the highest level, between the respective heads of state, that have governed Israel's nuclear conduct ever since.

That so little is known today about the tale of NSSM 40 is not surprising. Dealing with Israel's nuclear ambitions was thornier for the Nixon administration than for its predecessors because it was forced to deal with the problem at the critical time when Israel appeared to be crossing the nuclear threshold.

Yet, even as Nixon and Kissinger enabled Israel to flout the NPT, NSSM 40 allowed them to create a defensible record. As was his typical modus operandi, Kissinger used NSSM 40 to maintain control over key officials who wanted to take action on the problem.

Politically, the Nixon-Meir agreement allowed both leaders to continue with their old public policies without being forced to openly acknowledge the new reality. As long as Israel kept the bomb invisible -- no test, declaration, or any other act displaying nuclear capability -- the United States could live with it.

Over time, the tentative Nixon-Meir understanding became the foundation for a remarkable U.S.-Israeli deal, accompanied by a tacit but strict code of behavior to which both nations closely adhered. Even during its darkest hours in the 1973 Yom Kippur War, Israel was cautious not to make any public display of its nuclear capability.

Yet set against contemporary values of transparency and accountability, the Nixon-Meir deal of 1969 now stands as a striking and burdensome anomaly. Israel's nuclear posture is inconsistent with the tenets of a modern liberal democracy. The deal is also burdensome for the United States, provoking claims about double standards in U.S. nuclear nonproliferation policy.

It is especially striking to compare the Nixon administration's stance toward Israel in 1969 with the way Washington is trying to accommodate India in 2006. As problematic as the proposed nuclear pact with New Delhi is, it at least represents an effort to deal openly with the issue.

Unlike the case of Iran today -- where a nation is publicly violating its NPT obligations and where the United States and the international community are acting in the open -- the White House in 1969 addressed the Israeli weapons program in a highly secretive fashion. That kind of deal-making would be impossible now.

Without open acknowledgment of Israel's nuclear status, such ideas as a nuclear-free Middle East, or even the inclusion of Israel in an updated NPT regime, cannot be discussed properly. It is time for a new deal to replace the Nixon-Meir understandings of 1969, with Israel telling the truth and finally normalizing its nuclear affairs.

Avner Cohen is a senior research fellow at the Center for International and Security Studies at the University of Maryland and author of "Israel and the Bomb" (Columbia University Press). William Burr is a senior analyst at the National Security Archive at George Washington University. A longer version of this article appears in the May/June issue of theBulletin of the Atomic Scientists.